1. Field of the Invention
The present invention relates to a zoom lens system, and more specifically relates to a zoom lens system suitable for video cameras, digital cameras and the like.
2. Description of the Related Art
In recent years, demand has grown for zoom lens systems used in video cameras and digital cameras, of the type provided with solid-state sensing elements such as charge-coupled devices (CCD) and the like, which are compact and inexpensive while providing high performance. Generally, in zoom lens systems, compactness is achieved by increasing the optical power of the various lens elements while reducing the amount of movement of these lens elements. The higher degree of aberration generated by this arrangement can be suppressed to a desired value by using a lens having an aspherical surface. At least two or more lens elements are required to be effective to correct chromatic aberration, however, since an aspherical surface is unable to correct chromatic aberration.
The CCD format size, on the other hand, has tended to become more compact over the years with a corresponding demand for ever greater compactness in the size of the overall lens system. When the overall lens system is made more compact, however, there is a corresponding increase in sensitivity to manufacturing errors. Particularly in lenses comprising a plurality of lens elements, there is an increase in sensitivity to loss of performance characteristics in lenses due to spatial errors and decentering errors.
If lenses can be constructed using fewer lens elements, not only is compactness readily achieved, but cost reduction and ease of manufacture also can be realized. Therefore, although a lens required to correct chromatic aberration must be constructed using a minimum number of lens elements, there is a limitation on the optical system using only aspherical surfaces inasmuch as an aspherical surface is unable to correct chromatic aberration.
It has been proposed to reduce the number of lens elements by combined use of aspherical surfaces and lens elements of the refractive index profile type. Lenses of the refractive index profile type have the ability to correct all aberrations including chromatic aberration, and are extremely effective lenses having few lens elements. There also have been various proposals to correct chromatic aberration by combining diffractive optical elements and refractive optical elements.
For example, Japanese Laid-Open Patent Application No. 6-324262 discloses a photographic lens system using diffractive optical elements. Furthermore, Japanese Laid-Open Patent Application No. 5-164965 discloses a lens system having very few lens elements in a two-element zoom lens system of the refractive type. In two-component zoom lens systems of the positive-negative type, the moving lens element comprises a single refractive optical lens element.
When a lens of the refractive index profile type is used to correct chromatic aberration, there is negative dispersion distribution due to the extreme difficulty of manufacture. In the constructions disclosed in the aforesaid Japanese Laid-Open Patent Application Nos. 6-324262 and 5-164965, inexpensive lens elements with affixed films and the like are used in the zoom lens system, such that adequate performance cannot be assured relative to the photographic optical system using a CCD or the like as a sensing element.